The present invention relates to a ceramic coating formed on an iron tubular member for use in exhaust equipment of internal engines, etc. and a method of producing it.
For iron tubular members such as exhaust equipment of internal engines, etc., which are exposed to corrosive gases at high temperatures and severe heat shock, it was proposed to form ceramic coatings on the inner surfaces of such iron tubular members to impart them a heat resistance, a corrosion resistance and a heat shock resistance.
Big problems with such ceramic coatings are that since they are subjected to severe heat shock by a high-temperature exhaust gas, a large stress is generated in the boundaries between the ceramic coatings and the iron tubular members due to the differences in thermal expansion between them, leading to the peeling of the ceramic coatings from the iron tubular members, and that since the ceramic coatings have much smaller heat conductivity than the iron tubular members, an extremely large temperature gradient appears in the ceramic coatings by heat, thereby generating a large stress in the ceramic coatings, which leads to the peeling and cracking of the ceramic coatings.
In general, although ceramics have large compression strength, they have poor tensile strength and are extremely brittle. Accordingly, they are extremely less resistant to thermal shock.
To solve these problems, various proposals were made.
For instance, Japanese Patent Laid-Open No. 58-51214 discloses exhaust gas equipment for internal engines comprising an iron equipment body to be exposed to a high-temperature exhaust gas, an inner surface of which is coated with a refractory layer composed of a mixture of refractory material particles and a heat-resistant inorganic binder.
Japanese Patent Laid-Open No. 58-99180 discloses a method of producing exhaust gas equipment for internal engines which comprises the steps of forming a heat-resistant layer by coating an inner surface of an iron equipment body to be exposed to a high-temperature exhaust gas with a slip composed of a mixture of refractory material particles, an inorganic binder and frit: forming a, heat-insulating layer by coating the heat-resistant layer while it is in a wet state, with, heat-insulating particles: and then, after solidifying the heat-insulating layer, forming a heat-resistant layer thereon by coating the, heat-insulating layer with a slip composed of a mixture of refractory material particles, an inorganic binder and a frit. If necessary, the heat-resistant layer can be coated with a subsequent refractory, heat-insulating layer, and a subsequent heat-resistant layer repeatedly to produce a ceramic coating of a desired thickness.
However, these methods fail to provide sufficient bonding strength between the ceramic layers and the metal members, leaving the problem that ceramic layers are likely to peel off from the metal members along the bonding boundaries or in the ceramic layers themselves by heat shock. Thus, they are not satisfactory in long-period durability.
Recently, ceramic paints and coating materials containing metallic alkoxides as binders were developed. However, these materials are extremely expensive, and it is difficult to coat them in sufficient thickness for enabling them to endure use for a long period of time.
Further, Japanese Patent Laid-Open No. 59-12116 discloses a composite ceramic material comprising an inorganic hollow particles dispersed in a ceramic matrix. However, mere dispersion of inorganic hollow particles in a matrix fails to provide a coating having good bonding strength to a metal surface and high heat shock resistance, though it has sufficient heat insulation. In addition, since the inorganic hollow particles have small strength, they are easily broken, leading to the peeling and cracking of the resulting ceramic coating.
Recently, it has been found that when a ceramic coating bonded to an iron member is exposed to a corrosive exhaust gas, etc. at a high temperature for a long period of time, the corrosive exhaust gas penetrates into a ceramic layer and reaches to the boundary with the iron tubular member, thereby oxidizing the surface of the iron tubular member. Since the oxidation of the surface of the iron member generates cracks in the oxidized layer, the ceramic coating peels off easily by a mechanical shock or a heat shock. In addition, iron oxide is diffused into the coating layer, thereby causing the discoloration (blackening) of the resulting coating.